Effect of a Thermal Catalyst on Organosilanes Treatment to Improve Durability and Stability of Canadian Wood

Abstract

The demand for exterior wood siding is stagnating in North America due in part to perceptions of low durability and the need for frequent maintenance. One way to address these concerns is to modify the wood to improve its physical properties, while maintaining its appearance. In this study, white spruce was treated with organosilanes and a combination of aluminum treatments followed by a thermal treatment to improve the dimensional stability and the wood durability. Anti-swelling efficiency (ASE), leaching and decay tests were performed on the treated Canadian wood species. The quantity of hydroxyls available after treatment was evaluated by water vapor sorption. The results showed that the treatment improved the dimensional stability up to 50%. Available hydroxyls decreased by as much as 37%. The organosilanes treatment was resistant to leaching, while the aluminum was observed to leach. Organosilanes in combination with aluminum showed brown rot resistance. The addition of aluminum to the organosilanes treatment did not have effect on dimensional stability but it had a great impact on the brown rot resistance.

1. Introduction

The service life of wood products used in exterior applications is based on both functional criteria, such as mechanical performance, and aesthetic criteria, such as checking, warping, or surface discoloration. It is often the aesthetic criteria that define the product’s service life [1]. While maintaining resistance to biodegradation is essential for maintaining mechanical performance, different approaches are needed to extend aesthetic service life. Dimensional stability can be improved via physical approaches such as kerfing or profiling, which creates cuts on the back of a board, and by profiling which creates a series of troughs along the major face of a board, to reduce stresses induced by changing moisture content [2,3]. Similarly, coatings, water repellents and surface treatments have been reported to improve stability [4,5]. Various thermal and chemical modification technologies have also been used to improve dimensional stability. Thermal modification can improve dimensional stability by reducing the hydroxyl content [6]. Chemical treatments such as densification, can make wood more stable by filling the lumens with small molecules that can polymerize in situ and block accessibility to the hydroxyl groups. Some of these treatments (Kebony^® and Belmadur^®) have been commercialized mostly on permeable wood species such as radiata pine (Pinus radiata D. Don). Chemical reactions, such as acetylation, between wood hydroxyl groups and small molecules is another way to increase wood stability. Accsys Technologies has patented [7] and commercialized acetylated wood under the name Accoya^®. Although this treatment is only commercialized on radiata pine, a study reports dimensional stability increases up to 64% for acetylated spruce using microwave treatment [8]. Other molecules have been studied for their reaction with wood and potential dimensional stability improvement such as maleic anhydride [9], citric acid and glycerol [10], citric acid and sorbitol [11,12], and silanes [13,14]. Buffered amine oxide treatments applied to white spruce have also shown up to 30% of dimensional stability improvement [15].

There are many organosilanes, ranging from the simplest, such as methyltrimethoxysilane (MTMS) which is easier to impregnate, to organosilanes with long carbon chains, such as hexadecyltrimethoxysilane (HDTMS), which is usually used to increase surface material hydrophobicity. Depending on the organosilanes and their form (non-hydrolyzed or hydrolyzed), reported properties of treated wood are different. A study by Donath et al. [14] reports that hydrolyzed silanes favored lumen filling while monomeric organosilanes were found in the cell walls with a lower mass gain. Dimensional stability of Scots pine (Pinus sylvestris L.) and European birch (Fagus sylvatica L.) sapwood was improved by up to 30% when impregnated with tetraethylorthosilicate (TEOS), triethoxymethysilane (PTEOS), and methyltryethoxysilanes (MTES). Another study evaluated the ageing time of organosilanes treatment for the hydrolysis and condensation in the wood cell wall. They proved that the more the organosilanes are hydrolyzed, the less they will react with the wood cell wall. They also explained that the molecule size is important for the penetration of the cell walls [16]. Another study presented the presence of chemical bonding between silica and wood after two treatments. First the Black pine (Pinus nigra L.) was heated to 180–200 °C and in second step after grounding the heated wood into powder, chemical treatment was done with an organosilane. The conclusion of this article presented that chemical bonding was observed between the silica and the wood [17]. Elm wood (Ulmus spp.) presented anti-shrink efficiency (ASE) of 81% and 98% when impregnated with MTMS and (3-mercaptopropyl)trimethoxysilane (MPTES), respectively [18]. Previous work investigated the potential of an organosilanes treatment to improve the dimensional stability of Canadian refractory wood species using MTMS in acidic r basic conditions. Results obtained with this treatment were interesting as an increase in dimensional stability to humidity up to 30% was observed [19,20,21].

One of the hypotheses raised for the organosilanes treatment optimization was that increasing the temperature during the thermal treatment would further improve dimensional stability. To do so, a thermal catalyst such as aluminum sulfate (Al₂(SO₄)₃) could be used. The thermal treatment should increase hydroxyl condensation between organosilanes and wood cell wall hydroxyls. This would result in a non-reversible condensation. Qu et al. reported that the use of aluminum sulfate as a pretreatment compared with thermal treatment alone increased the dimensional and thermal stability of wood, allowing thermal treatment to be performed at a lower temperature [22]. In the present work, after impregnation, half of the treated wood samples were impregnated with an aluminum sulfate solution. The goal of this additional step was to use the aluminum as a thermal catalyst to accelerate condensation of the organosilanes hydroxyls during thermal treatments (120 and 140 °C). The present work aims to evaluate the performance of refractory Canadian species, white spruce (Picea glauca (Moench) Voss) and Jack pine (Pinus banksiana Lamb.), treated with an optimized organosilanes treatment.

2. Materials and Methods

2.1. Materials

Oven dried wooden boards from a mix of heartwood and sapwood of white spruce (Bois d’oeuvre Cedrico, Quebec, QC, Canada) and Jack pine (Résolu, La dorée, QC, Canada) were cut into two different sized blocks: 19 × 19 × 19 mm³ (L × T × R) and 50 × 50 × 19 mm³ (L × T × R) without any knots or defects. Specimens were paired to have control and differently treated samples from the same board for each replicate. Methyltrimethoxysilane (>98%), hexadecyltrimethoxysilane (95%) were provided by Gelest Inc. (Morrisville, PA, USA) and glacial acetic acid (>99%), ethyl alcohol and aluminum sulfate (>97%) were provided by Sigma Aldrich (Toronto, ON, Canada).

2.2. Solutions Preparation and Wood Treatment

First, methyltrimethoxysilane (MTMS) was mixed with ethyl alcohol in a 6:1 molar ratio for 5 min at 60 °C. Then, 1% (wt/wt) of glacial acetic acid was added slowly to the solution. The solution was stirred at 60 °C for 30 min. Finally, 6.2% of hexadecyltrimethoxysilane (HDTMS) was added and the solution was stirred for 60 min. For the aluminum aqueous solution, the solution was prepared with aluminum sulfate in distilled water. The aluminum solution was prepared in distilled water with a concentration of 0.44 mol·L⁻¹ of aluminum sulfate.

Wood specimens were conditioned at 20 °C and 80% relative humidity until the mass was stabilized. The resulting average moisture content (MC) was 16%. Specimens were impregnated under vacuum pressure treatment with the organosilanes solution according to the parameters presented in

Table 1. Parameters used for impregnation and thermal treatment depending on samples size.

Treatment Parameters	19 mm × 19 mm × 19 mm Specimens	50 mm × 50 mm × 19 mm Specimens
Vacuum (85 kPa)	15 min	30 min
Pressure (620 kPa)	45 min	2 h
Ethanol evaporation	Min. 12 h	Min. 12 h
Oven temperature (°C)	120 or 140	120 or 140
Time in oven (h)	17	24

. Half of the organosilanes treated samples subsequently were impregnated with an aqueous aluminum sulfate solution with the same treatment parameters. After the treatment, samples were air-dried for 12 h at ambient conditions. Then, organosilanes treated samples, with and without aluminum treatment, were heated at 120 or 140 °C for 17 or 24 h, depending on the specimen size (Table 1). Treated samples were conditioned at 20 °C and 50% of relative humidity until the mass were stabilized.

For fungal resistance tests, twenty-eight white spruce and jack pine blocks (19 mm × 19 mm × 19 mm) were pressure treated with alkaline copper quaternary (ACQ-C) for inclusion as a negative control. The treatment schedule included an initial 30 min vacuum at 95 kPa, 60 min pressure at 414 kPa, and a final 15 min vacuum at 95 kPa. A solution strength of 0.46% ACQ-C was used.

2.3. Dimensional Stability

Table 2. Identification and description of the different treatment performed on white spruce and jack pine.

Species	ID	Treatment
White spruce (EB)	EB 120	TT at 120 °C
	EB 140	TT at 140 °C
	EBA 120	MTMS + HDTMS treatment (A) + TT at 120 °C
	EBA 140	MTMS + HDTMS treatment (A) + TT at 140 °C
	EBAL 120	MTMS + HDTMS treatment (A)+ Al treatment (L)+ TT at 120 °C
	EBAL 140	MTMS + HDTMS treatment (A)+ Al treatment (L)+ TT at 140 °C
	EBTAL 120	Al treatment (TAL) + TT at 120 °C
Jack pine (PG)	PG 120	TT at 120 °C
	PG 140	TT at 140 °C
	PGA 120	MTMS + HDTMS treatment (A) + TT at 120 °C
	PGA 140	MTMS + HDTMS treatment (A) + TT at 140 °C
	PGAL 120	MTMS + HDTMS treatment (A) + Al treatment (L) + TT at 120 °C
	PGAL 140	MTMS + HDTMS treatment (A) + Al treatment (L) + TT at 140 °C
	PGTAL 120	Al treatment (TAL) + TT at 120 °C

TT: thermal treatment. Al: aluminum.

presents the different treatments performed for the evaluation of dimensional stability compared to control samples for each species.

For the humidity test, specimens of 50 mm × 50 mm × 19 mm (15 for each series) were placed in a chamber at 20 °C/50% RH until mass stabilization (<0.2% in 24 h). Mass and radial, tangential, and longitudinal dimensions were measured using a balance and calipers. Subsequently, the samples were placed in a Z-plus conditioning chamber (Cincinnati sub-zero, Cincinnati, OH, USA) at 20 °C/90% RH. When the sample mass stabilized (<0.2% in 24 h), the mass, radial, tangential and longitudinal dimensions were measured using a balance and calipers. This cycle was repeated up to three times.

For the immersion test, specimens of 50 mm × 50 mm × 19 mm (15 for each series) were placed in a chamber at 20 °C/50% RH until mass stabilization (<1% in 24 h). Mass, radial, tangential, and longitudinal dimensions were measured using a balance and calipers. Subsequently, the samples were immersed in water at 22 °C. When the samples showed small variations in mass (<1% in 24 h), the mass, radial, tangential, and longitudinal dimensions were measured using a balance and calipers. The cycle was repeated up to three times.

Dimensional stability was determined by calculating volume swelling and anti-swelling efficiency (ASE_v). The ASE_v is calculated from the swelling coefficients of treated and untreated samples subjected to different conditions according to the equations from DIN 52184 [23]:

$${\mathrm{Sw}}_{\mathrm{ctrl}}=\left(\frac{{\mathrm{V}}_{\mathrm{ctrl}90\mathrm{ou}}{\mathrm{V}}_{\mathrm{ctrlimm}}-{\mathrm{V}}_{\mathrm{ctrl}50}}{{\mathrm{V}}_{\mathrm{ctrl}50}}\right)\times 100$$

(1)

$${\mathrm{Sw}}_{\mathrm{tr}}=\left(\frac{{\mathrm{V}}_{\mathrm{tr}90\mathrm{ou}}{\mathrm{V}}_{\mathrm{trimm}}-{\mathrm{V}}_{\mathrm{tr}50}}{{\mathrm{V}}_{\mathrm{tr}50}}\right)\times 100$$

(2)

$${\mathrm{ASE}}_{\mathrm{v}}\left(\%\right)=\left(\frac{{\mathrm{Sw}}_{\mathrm{ctrl}}-{\mathrm{Sw}}_{\mathrm{tr}}}{{\mathrm{Sw}}_{\mathrm{ctrl}}}\right)\times 100$$

(3)

where V_ctrl90 is the volume of untreated sample at 90% RH, V_ctrlimm is the volume of untreated sample after immersion in water; V_ctrl50 is the volume of untreated sample at 50% RH, V_tr90 is the volume of treated sample at 90% RH, V_trimm is the volume of treated sample after immersion in water; V_tr50 is the volume of treated sample at 50% RH.

2.4. Hydroxyl Quantification by Water Vapor Sorption

The hydroxyl accessibility of the samples was measured with a vapor sorption analyzer (VTI-SA+, Ta Instrument, New Castle, DE, USA) according to the method of Thybring et al. [24]. The measurement sequence consisted of three steps all performed at 24 °C ± 0.5 °C. First, 5 to 10 mg of wood sample was dried at 0% RH with a flow rate of 200 cm³/min until the mass was stabilized. Then, the relative humidity of the measurement chamber was increased with D₂O vapor (99.9 atom% D, Sigma Aldrich, St. Louis, MO, USA) and held constant for 300 min. To complete the measurement sequence, the specimens were dried again in the same manner as the starting step. The OH accessibility was calculated as follows and the reduction of OH accessibility was calculated.

$${\mathrm{OH}}_{\mathrm{a}}=\frac{\mathrm{mf}-\mathrm{mi}}{\mathrm{mi}\ast \left(\mathrm{MD}-\mathrm{MH}\right)}\times 1000$$

(4)

where OH_a are the accessible hydroxyls in percent (%) in the anhydrous mass of the product; mi is the initial anhydrous mass of the sample before exposure to D₂O (g); mf is the final anhydrous mass of the sample after exposure to D₂O (g); MD is the molecular weight of deuterium (2.014 g/mol); and MH is the molecular weight of hydrogen (1008 g/mol).

2.5. Leaching Test

Leaching tests were performed on three specimens (19 mm × 19 mm × 19 mm) per treatment. The tests were adapted from the AWPA E 11 Standard Method for Accelerated Evaluation of Preservative Leaching [25].

The samples were placed in a chamber at 20 °C/50% RH until constant mass was reached. Afterwards, the samples were placed in different containers filled with distilled water and placed for 5 min under vacuum (85 kPa) and then immersed in 150 mL of distilled water for 2 weeks. The leachates were collected and stored at 4 °C. The water was changed after 1 day, 4 days, 10 days, and 14 days. The leachate and a distilled water sample were sent to Environex Laboratory (Quebec, QC, Canada) for analysis by Inductively coupled plasma atomic emission spectroscopy (ICP-AES) to determine the amount of silicon and aluminum present in the leachate.

2.6. Brown Rot Resistance

Brown rot resistance tests were performed on unleached 19 mm × 19 mm × 19 mm cubes. Each treatment group included six replicates for each fungal strain as well as two non-inoculated control blocks. Treatments ID and descriptions are presented in

Table 3. Treatment ID and their description tested against brown rot.

Treatment ID	Description
EBNT	Untreated white spruce
EBA	White spruce + organosilanes
EBAL	White spruce + organosilanes + aluminum sulfate
EBACQ	White spruce treated with ACQ-C
PGNT	Untreated jack pine
PGA	Jack pine + organosilanes
PGAL	Jack pine + organosilanes + aluminum sulfate
PGACQ	Jack pine treated with ACQ-C
PP sapwood	Untreated Ponderosa pine sapwood

. Untreated Ponderosa pine (Pinus ponderosa Douglas ex C. Lawson) sapwood blocks were included as positive controls. Blocks were conditioned overnight at 40 °C in a forced-draft oven and then left in a constant humidity and temperature room (CTH) for 48 h prior to weighing. Test blocks were then sent to Iotron industries (Port Coquitlam, BC, Canada) for sterilization by 25 kiloGrays of ionizing irradiation before installation in soil jars with selected fungi.

The test method followed AWPA EI0-16 Laboratory Method for Evaluating the Decay Resistance of Wood-Based Materials Against Pure Basidiomycete Cultures: Soil/block Test (AWPA, 2022) [26] as described with the following modifications to the soil. A commercial horticulture loam with a water holding capacity of approximately 80% was used. The soil moisture content was adjusted to the point where the soil just clumps when squeezed manually. For the brown rot sample jars, two feeder strips of 3 mm × 29 mm × 35 mm pine sapwood were placed in each jar. The jars were sterilized at 120 °C and 15 kPa for 60 min, cooled and then each feeder strip was inoculated under sterile conditions with one of the brown rot fungi presented in

Table 4. Fungi strain used for the brown rot resistance test.

Rhodonia placenta (Fr.) M. Lars. et Lomb.	Ftk 120F
Gloeophyllum trabeum (Pers. Ex FR.) Murr.	Ftk 47D
Fibroporia radiculosa (Peck) Gilb. & Ryvarden	TFFH 294

.

The three fungi used in this test are known to cause decay in wood. Following inoculation, soil jars were incubated for 4 weeks at conditions as outlined in the standard. Sterilized blocks were aseptically planted, two to each jar, one on each of the feeder strips once mycelia had completely covered each strip. The soil jars were replaced in the conditioning chamber and held for 12 weeks. After 12 weeks in test, blocks were removed from each jar, wiped of any adhering mycelium, and the wet weights was recorded. Blocks were conditioned at 40 °C in a forced-draft oven to a constant weight and then placed in the CTH room for 48 h prior recording post-conditioned weights. Block moisture content and weight losses at the end of test were calculated based on the initial conditioned weight, the end of test wet weight, and the post-conditioned weight. Corrected weight losses were calculated using non-inoculated check block weight differences from the end of test compared to the initial weights to compensate for any non-fungal mass loss or gains that might occur during incubation. Moisture content percentage as well as mass losses percentage were calculated for all test samples.

3. Results

3.1. Retention and Weight Percant Gain (WPG)

Figure 1 shows, from left to right, the control wood samples, the wood samples treated with organosilanes alone and the wood samples treated with organosilanes and aluminum. The color of the samples is slightly browner for the organosilanes alone compared to the control due to the thermal treatment at 120 °C. The aluminum and organosilanes treated wood samples are completely brown due to the presence of aluminum which is used as a heat catalyst.

Treatment retention (organosilanes (OSi) or aluminum sulfate (Al)) before thermal treatment and the weight percent gain after thermal treatment, for the two species are presented in

Table 5. Retention after impregnation and weight percent gain after thermal treatment for white spruce and jack pine (50 mm × 50 mm × 19 mm samples).

-	White Spruce				Jack Pine
-	120 °C (18 h)		140 °C (18 h)		120 °C (18 h)		140 °C (18 h)
-	EBA	EBAl	EBA	EBAl	PGA	PGAl	PGA	PGAl
OSi retention before thermal treatment (%)	79	84	79	84	54	58	52	50
Al retention before thermal treatment (%)	-	36	-	36	-	36	-	34
WPG after thermal treatment and stabilization at 20 °C/50% RH	11	12	11	12	8	8	7	4

. Organosilanes retention was between 79% and 84% for white spruce, and between 50% to 58% for jack pine. The retention of the aluminum sulfate aqueous solution, after impregnation with the organosilanes, was between 34% and 36%, regardless of the species. The weight percent gain after thermal treatment was 11% to 12% for white spruce and between 4% to 8% for jack pine. Thermal treatment at 140 °C shows the smallest weight percent gain (4% and 7%) for jack pine.

3.2. Dimensional Stability

3.2.1. Dimensional Stability under Humidity Cycling

Figure 2 shows the anti-swelling efficiency (ASE_v) to humidity for the different treatments on white spruce and jack pine (refer to Table 2). The thermal treatments alone, used on white spruce and jack pine, exhibited low anti-swelling efficiency (<20%). Only after the third cycle, for the thermal treatment at 140 °C, on white spruce, was an ASE_v of 34% is obtained. Treatments with organosilanes alone or organosilanes + aluminum sulfate showed poor ASE_v values after the first cycle. After the second cycle, ASE_v values obtained for white spruce were between 27% and 37%, while values for jack pine were between 18% and 43%. Furthermore, there was no decrease in the efficiency of the treatments after three cycles in the presence of humidity. Increasing temperature or using aluminum sulfate in addition to the organosilanes treatment did not improve ASE_v values. The ASE_v associated with the aluminum sulfate and thermal treatment without organosilanes was as high as with organosilanes treatment and, even after the third cycle, values higher than 50% were obtained for both species.

3.2.2. Dimensional Stability under Water Immersion

Figure 3 shows the anti-swelling efficiency under water immersion for the different treatments on white spruce and jack pine. In general, the ASE_v values obtained for the immersion test were lower than the values obtained for the humidity. The thermal treatments alone showed no improvement in the ASE_v values, and a decrease in ASE_v values was found for jack pine. Organosilanes treatments alone or with aluminum treatment showed low ASE_v values (<30%), when immersed, for both species studied. The highest ASE_v values obtained were for the aluminum and thermal treatments, EBAL140 and PGAL120 with values higher than 30%. Aluminum sulfate treatment alone presents promising results similar to the ASE_v values obtained with EBAL120 and PGAL140.

3.3. Hydroxyl Quantification by Water Vapor Sorption

Table 6. Percentage decrease in accessible OH between treated and untreated wood samples for the methods WVS (water vapor sorption).

-	EBA 120	EBA 140	EBAl 120	PGA 120	PGA 140	PGAl 120
Decrease in OH accessibility (WVS)	33%	21%	37%	15%	17%	17%

shows the percentage decrease in accessible hydroxyls for organosilanes treated and aluminum sulfate treated, white spruce and jack pine, compared to untreated wood. One general observation is that the treatments were more efficient on white spruce than jack pine to decrease OH accessibility which is aligned with the dimensional stability and weight percent gain results obtained. For jack pine, percentage was similar regardless of the treatment used. For white spruce, the most efficient treatment to decrease OH accessibility was the EBAl 120.

3.4. Leaching Test

Figure 4 and Figure 5 show the percent of cumulative leaching of silicon and aluminum after 1, 4, 7, and 14 days of leaching test for the different treatments. For all treated samples, less than 5% of silicon leached after 14 days of leaching. Leaching was less for wood treated at a higher temperature (140 °C), regardless of species (white spruce or jack pine). Leaching for jack pine was lower than for white spruce, but this may be explained by the lower mass gain obtained for jack pine. For all samples, silicon appears to continue to leach out even in small amounts throughout the test. Leaching tests of aluminum-treated spruce and jack pine (EBAl and PGAl) show a 20%–25% loss of aluminum from the first day. After 14 days, nearly half of the aluminum has leached out, regardless of species. Additionally, the aluminum concentration in the leachate is slightly higher for jack pine than for white spruce during the first week of testing.

3.5. Brown Rot Resistance

The moisture contents at the end of incubation and corrected weight losses for test samples are summarized in

Table 7. Average values and standard deviations for moisture contents and corrected weight losses for test blocks exposed to Rhodonia placenta after 12 weeks of incubation.

Product ID	Treatment ID	Moisture Content (%)		Corrected Weight Loss (%)
		AVG	STD	AVG	STD
White Spruce	Untreated	236.7	33.0	59.3	7.0
White Spruce	Si	97.5	20.4	48.8	6.7
White Spruce	Si + Al	100.4	16.5	5.8	2.9
White Spruce	ACQ-C	39.4	4.8	0.3	0.1
Jack pine	Untreated	220.0	15.9	51.5	5.6
Jack pine	Si	147.2	26.3	51.6	2.2
Jack pine	Si + Al	96.7	11.3	1.8	0.9
Jack pine	ACQ-C	38.3	2.3	0.0	0.0
Ponderosa Pine sapwood	Untreated	188.0	42.0	57.7	5.5

,

Table 8. Average values and standard deviations for moisture contents and corrected weight losses for test blocks exposed to to Gloeophyllum trabeum after 12 weeks of incubation.

Product ID	Treatment ID	Moisture Content (%)		Corrected Weight Loss (%)
		AVG	STD	AVG	STD
White Spruce	Untreated	224.1	54.9	46.1	4.1
White Spruce	Si	86.4	41.5	23.9	4.9
White Spruce	Si + Al	91.6	31.1	1.8	0.8
White Spruce	ACQ-C	31.9	3.1	0.1	0.3
Jack pine	Untreated	192.3	34.8	49.3	6.4
Jack pine	Si	117.4	36.2	34.2	3.5
Jack pine	Si + Al	65.8	8.0	1.0	1.1
Jack pine	ACQ-C	31.3	3.9	0.0	0.1
Ponderosa Pine sapwood	Untreated	167.1	20.6	59.6	4.6

and

Table 9. Average values and standard deviations for moisture contents and corrected weight losses for test blocks exposed to Fibroporia radiculosa after 12 weeks of incubation.

Product ID	Treatment ID	Moisture Content (%)		Corrected Weight Loss (%)
		AVG	STD	AVG	STD
White Spruce	Untreated	233.5	15.1	52.2	4.2
White Spruce	Si	109.9	48.7	28.3	9.4
White Spruce	Si + Al	95.7	14.8	1.6	0.9
White Spruce	ACQ-C	37.3	4.3	0.2	0.1
Jack pine	Untreated	194.6	22.8	35.4	8.5
Jack pine	Si	152.4	30.1	33.8	9.7
Jack pine	Si + Al	100.1	10.6	1.2	0.4
Jack pine	ACQ-C	35.7	1.9	0.1	0.1
Ponderosa Pine sapwood	Untreated	195.2	53.6	50.7	9.7

for the brown rot evaluated. Minor weight losses or gains were observed in non-inoculated checks blocks. These differences have been used as a correction factor to calculate weight losses for each treatment group. High weight losses (35% to 59%) were found in all untreated groups exposed to all three fungi. This confirms that the test conditions supported vigorous decay. Moreover, it confirms that the unmodified white spruce and jack pine material is susceptible to decay. The ACQ-C treated blocks (negative control) were unaffected by the decay fungi with average weight losses of less than 1%. The organosilanes treatment without aluminum showed minimal effect on decay resistance with weight losses ranging from 24% to 52%. However, the organosilanes treatments with aluminum showed significant improvement in decay resistance with weight losses ranging from 1% to 6%, with only the treated white spruce exposed to R. placenta exceeding the 3% weight loss threshold. A final weight loss greater than 3.0% indicates failure of the preservative system to protect the wood from decay by the inoculation fungus. The three percent limit is commonly used [27] because non-wood-rotting fungi can break down up to three percent of the non-structural components of wood.

4. Discussion

In the organosilanes treatment, 25% (wt/wt) of hexadecyltrimetoxysilane was added to improve the hydrophobicity of the organosilanes treatment while maintaining the other 75% as small molecules with methyltrimethoxysilane. This mix of organosilanes was based on different studies that reported that molecular weight of the organosilanes and their hydrolyzed state influence where they will be located into the wood. Indeed, hydrolyzing organosilanes before impregnation will increase organosilanes’ reaction in the lumens rather than in the wood cell wall compared to non-hydrolyzed organosilanes that will penetrate more easily the wood cell wall due to their size [14]. As discussed previously [19,20,21], the non-hydrolyzed organosilanes penetrated the wood cell wall while avoiding filling the lumen, and the organosilanes were chemically bonded to the wood after the thermal treatment. After organosilanes treatment and organosilanes and aluminum treatment on white spruce and jack pine, some differences were observed with mass gain between 5 and 12%. The low WPG for jack pine compared to white spruce is also explained because of a lower organosilanes solution retention compared to the white spruce. This can be explained by its lower permeability [28], or possibly the presence of heartwood as the log diameter was small.

Anti-swelling efficiency percentages obtained in the humidity test, after the second cycle, demonstrate that organosilanes treatment alone and organosilanes + aluminum sulfate can improve dimensional stability. However, after the first cycle, treatments with organosilanes alone or organosilanes + aluminum sulfate showed poor ASE_v values. The hypothesis is that, initially, there was unreacted organosilanes in the wood which provided additional available hydroxyl groups. Subsequently, after the first cycle, either due to leaching or due to the continuous reactions between organosilanes and wood hydroxyls, the ASE_v improved in the second and third cycle. The improved ASE_v value is consistent with previous work showing that non-hydrolyzed organosilanes penetrated the wood cell wall and few lumens [19,20]. Increasing temperature during thermal treatment or using a thermal catalyst with organosilanes did not improve ASE values contrary to what was expected. However, aluminum sulfate treatment alone with a thermal treatment of 120 °C provided some interesting results, similar to the organosilanes treatment alone. The use of aluminum sulfate alone as a wood treatment should be further investigated. These results with the aluminum are in accordance with the results on Chinese fir reported by Qu et al. (2021) [22]. For the dimensional stability in immersion, the values obtained are quite low for all the treatments, between 15%–30%. It appears that organosilanes being present in the cell wall, are not enough to improve the ASE_v in immersion by more than 30%. The explanation could be that siloxy bonds formed due to the condensation reaction could undergo secondary hydrolysis when the organosilanes are insufficiently condensed to form a reticulated compound in the wood. That may explain the possible poor results in the immersion dimensional stability test.

Organosilanes treatment + aluminum sulfate, on white spruce, reported the highest decrease in percentage of hydroxyl groups available. It is, however, unclear if the decrease in OH availability is due to the increase in condensation between organosilanes and wood hydroxyls caused by the presence of aluminum sulfate or by the degradation of hemicellulose caused by the aluminum sulfate as a thermal catalyst [22]. There were only small differences in the decrease of available hydroxyls for jack pine treated samples compared to the untreated. This can be explained by the low WPG observed after thermal treatment for the jack pine treated samples.

After 14 days of leaching, less than 5% of the silicon leached from all treated samples, regardless of species or treatment, demonstrating that condensation occurred between the alkoxy groups and carbon of the wood polymers (Si-O-C). However, leaching continued to occur throughout the test. One hypothesis is that the siloxy bond created (Si-O-C) undergoes secondary hydrolysis which could also confirm the hypothesis observed for dimensional stability in immersion. On the other hand, the aluminum leachates were very high from the first day of the leaching test (20%–25%). This reached half of the aluminum leached from the treated wood after 14 days. This means that the aluminum compounds are not bound to the wood and are easily leached out. In fact, the aluminum was present for this treatment only as a thermal catalyst without reacting with the organosilanes or the wood.

The performance against three decay fungi for samples treated with organosilanes + aluminum sulfate showed significant improvement compared to untreated samples and samples treated with organosilanes alone. Unfortunately, this study did not include an aluminum sulfate alone treatment, which would have helped to interpret the data. It is unclear from the present work, whether the presence of aluminum was toxic to the decay fungi as aluminum compounds have been found to inhibit wood decay fungi [29,30], or whether the aluminum caused the organosilanes to react with wood in a different manner that led to increased decay resistance. There is also the possibility that the aluminum sulfate, being a thermal catalyst, had thermally modified the wood. It is reported that thermal treatment affects the chemical structure of the wood, mainly hemicellulose, which increases decay resistance [31]. This study did not investigate resistance to white rot fungi, as they are less common on softwoods, nor did it consider resistance to insects, which are less important in Canada. Data on the performance of these test materials against such biodegradation threats and under field conditions would be needed to understand the impact of these treatments on functional service life.

5. Conclusions

Optimized organosilanes treatment with a thermal treatment of 140 °C and organosilanes + aluminum sulfate treatments can improve dimensional stability of white spruce and jack pine. Moreover, organosilanes + aluminum treatments demonstrated resistance to three strain of brown rot fungi. Contrary to what was expected, the addition of a thermal catalyst (aluminum sulfate) did not improve the dimensional stability to humidity and to immersion in comparison to the organosilanes treatment alone, even if a decrease in the hydroxyls’ availability was observed. A low silicon leaching occurred for the treated wood during all the duration of the leaching test, suggesting possible secondary hydrolysis of the siloxy bonds. The aluminum, which did not react with treated wood, has leached over time. Based on the results obtained, further investigations are needed on the combined treatment organosilanes + aluminum and on the aluminum treatment alone to confirm their impact on wood stability and durability.